1. Field of the Invention
This invention relates to ion mobility spectrometers and more particularly to an ion mobility spectrometer having an increased sensitivity that is compact and simple in design.
2. Description of the Prior Art
The ion mobility mass spectrometer (abbreviated as IMS) is an electrostatic analytical instrument which separates ions in terms of their mobility. A typical spectrometer, such as the prior art device illustrated in FIG. 1, has two contiguous cylindrical regions called the reaction and drift regions that are kept under a steady flow of usually atmospheric pressure nitrogen or air. Ions created in the reaction region of the spectrometer are pulsed into the drift region. Here they see a uniform axial electric field of about 300 volts/cm which causes them to travel with different speeds according to their mobility in the ambient gas, thus separating them according to their mobility. The drift velocity Vd of the ions can be expressed according to the relations shown in equations (1) and (2): EQU Vd=KE (1) EQU K=Ko (T/273)(760/P) (2)
where E is the electric field seen by the ions, T is the absolute temperature and P the pressure in torr of the drift gas. The constant of proportionality K is called the mobility of the ions. The reduced mobility Ko takes into account variations in the mobility due to changes in temperature and pressure. The ions after reaching the end of the drift region impinge on a metal plate connected to a charge amplifier which gives a signal output proportional to the number of ions collected by the metal plate. The output from the amplifier consists of a series of voltage peaks separated in time, each peak corresponding to ions of a given mobility.
The reaction region has a weaker axial electric field, about 200 volts/cm. The reactant molecules are ionized in the reaction region using an electron source, usually a beta ray emitter like .sup.63 Ni The product ions drift towards the end of the reaction region where an electrostatic shutter 22, called the Neilson-Bradbury shutter, is used to pulse the ions into the drift region. In its closed position the shutter is capable of producing an electric field transverse to the direction of motion of the ions. This deflects the ions away from the axis and neutralizes them thus preventing them from entering the drift region. The shutter is open when this transverse electric field is removed. This is usually achieved by electronically shorting the alternate wires in the shutter grid structure. By removing the transverse electric field, ions are allowed to enter the drift region. Typically the shutter is opened for a duration of 200 microseconds every 20 milliseconds.